1. Field of the Invention
The present invention relates to a non-volatile semiconductor memory device and, more particularly to a read only memory (ROM) which is programmable and in which the stored data may be electrically erased by the user.
2. Description of the Prior Art
A conventional erasable ROM comprises a plurality of memory cells in a matrix form, each memory cell having a metal oxide semiconductor field effect transistor (MOSFET) which has an electrically insulated floating gate and a control gate overlaid on the floating gate. MOSFETs which are aligned in a row of the ROM are connected to a common word line. MOSFETs which are aligned in a column of the ROM are connected to a common bit line. When data is to be written into the EPROM, a certain row word line and a certain column bit line are selected in response to the data, so that a floating gate of a MOSFET for a desired memory cell is charged. The charging operation of the floating gate is performed by transferring electrons, for example, from the substrate to the floating gate according to tunneling or avalanche injection. The selected MOSFET is maintained in the logic "0" state or the logic "1" state in response to data.
On the other hand, when data stored in the erasable ROM is to be erased or cleared out for writing new data, short wavelength photoemission such as ultraviolet light is radiated on the erasable ROM. Floating gates of the MOSFETs which constitute the memory cells are discharged. As a result, data contents of all memory cells are simultaneously erased. Therefore, even when the erasable ROM is applied to a microprocessor-based system and partial erasure of a microprogram contained in the erasable ROM is desired, all memory contents of the erasable ROM must be cleared out. In this way, according to the conventional erasable ROM, partial or selective erasure of the stored contents cannot be performed.